The incentive to develop a cost-effective technology designed to reduce NO.sub.x emissions from all stationary combustion sources was provided in 1990 through the enactment by the U.S. Congress of the Clean Air Act Amendments (CAAA). A recent publication (EPA-453/R-94-022) by the Environmental Protection Agency (EPA) clearly indicates that the new NO.sub.x reducing timetable is to be applied to ICI boilers (which range in heat input size from 0.4 to 1,500 MBtu/hr), specifically those that emit over 25 tons of NO.sub.x per year. Such an annual NO.sub.x emission rate can be achieved, for example, by an oil burning furnace rated at 5 MW or a gas-burning furnace rated at 10 MW.
A number of methods are known for the reduction of NO.sub.x emissions in existing boilers. Modifications in the combustion process are usually the simplest and cheapest. Typical examples are a) to switch to a low N-containing fuel; b) to minimize the use of excess air; c) to lower the combustion temperature by injecting water or steam; d) to decrease the residence time in the flame zone; e) to recirculate and reburn all or part of the flue gases; f) to install an after burner; or g) to retrofit with a new low NO.sub.x burner. Other NO.sub.x reducing technologies involve some form of treatment of the flue gases. The three main sub-groups are a) the selective catalytic reduction (SCR) or the selective non-catalytic reduction (SNCR) reaction between NO.sub.x and ammonia (or urea) to N.sub.2 ; b) the wet scrubbing of the gas stream with an oxidant (to convert NO to the soluble NO.sub.2 or with a NO-specific absorber; or c) the creation of short lived but highly reactive free radicals by means of electrical discharge.
The retrofit technology currently most popular with owners of ICI boilers, in terms of NO.sub.x reduction, is the triple modification involving a change to natural gas, installation of a new low NO.sub.x burner, and recirculation of the flue gas for a reburn. The selective catalytic reduction installation (SCR) is also a popular flue gas treatment technology, despite its high installation costs. The SCR technology is currently recognized both by the industry and the EPA as the Best Available Technology (BAT) because it is able to reduce NO.sub.x emissions from both gas-fired and oil-fired boilers by over 80%.
Non-catalytic NH.sub.3 induced NO.sub.x reduction (SNCR) replaces SCR when coal is burned or when high-sulfur containing fuels are used. The main advantage to SNCR over SCR is that it is twice as cost effective even though it generally does not achieve NO.sub.x reductions better than 60%. Its main disadvantage is that it requires high flue gas temperatures, generally above 800.degree. C., whereas most ICI boilers are now equipped with economizers which reduce the flue gas temperature to 300.degree. C. or less.
The effect of applying an electrical discharge to flue gases, e.g. pulsed corona discharge, dielectric barrier discharge, DC glow discharge, E-beam, etc. has been researched with the intention to use the short lived free radicals produced thereby for the reaction with NO.sub.x molecules. That process reduces most of the NO.sub.x to N.sub.2 without also reducing the CO.sub.2 molecules to CO. However, installation and operating and maintenance costs appear to impede commercialization of that technology. It is a further disadvantage that the electrical discharge produces the OH radical (as well as O, H, O.sub.3 radicals) which reacts with CO and N.sub.2 to produce the NO, possibly leading to an increase rather than a decrease in NO.sub.x.
It is a major disadvantage of all the above-described measures and systems for the reduction of NO.sub.x emissions that they require either non-recurring or constant, non-recoverable capital expenditures. Thus, a more economical NO.sub.x emissions reduction method and system is desired.
The creation of oxygen radicals (O or O.sub.3) during the oxidation of yellow phosphorus (P.sub.4) has been common knowledge for many years (Thad D. Farr, Phosphorus. Properties of the Element and Some of its Compounds, Tennessee Valley Authority, Wilson Dam, Ala., Chem. Eng. Report #8 1950; J. R. Van Wazer, Phosphorus and its Compounds, Interscience, New York 1958). The suggestion to introduce P.sub.4 into the spray of a wet scrubber to oxidize NO to N.sub.2 in the flue gas stream from a boiler was made by S. G. Chang and G. C. Liu (Nature 343:151-3, 1990).
In U.S. Pat. No. 5,106,601 of Chang et al a wet scrubbing process for the removal of NO.sub.x from flue gases, wherein the flue gas is treated in a spray tower scrubber or a bubbling absorber with an aqueous emulsion of yellow phosphorus in water. The process is described as being operative throughout the liquid range of water with an optimal temperature range being 45.degree. C. (113.degree. F.) to 75.degree. C. (167.degree. F.) for the scrubbing emulsion. Although Chang et al. achieve significant reduction of NO.sub.x in the flue gas, the amount of scrubbing liquid and, thus, P.sub.4 required to achieve NO.sub.x reductions above those of the prior art processes mentioned above are relatively high. Since the cost of yellow phosphorus is significant ($1 per lb), the use of large amounts of scrubbing liquid is uneconomical and renders the wet scrubbing process of Chang et al. unpractical for large scale industrial applications. Thus, a flue gas treatment process is desired which would provide for significant reductions in NO.sub.x concentration without the use of large amounts of phosphorus.
The location of the P.sub.4 /NO interaction in the spray of the scrubber as suggested by Chang et al. makes it difficult to ensure total confinement of all undesirable reaction products, i.e. ozone and phosphorus oxides. Furthermore, the P.sub.4 /NO interaction in the liquid/gas interface in a wet scrubber makes it difficult to achieve the optimum P/N molar ratio necessary for cost effectiveness. Other problems with the suggested inclusion of P.sub.4 in the liquid spray of a wet scrubber are that the reservoir at the bottom of the wet scrubber cannot contain excess P.sub.4 if the reservoir is to be clarified or filtered and that the efficiency of the reaction between the oxygen radicals and the NO molecules is dependent on the residence time of the reactants in the spray tower. Finally, the reactions which take place between P.sub.4 and NO are believed to occur in the vapour phase. Thus, the inclusion of the phosphorus in the scrubbing liquid of a wet scrubber would significantly reduce the interaction between the reactant involved and result in longer treatment times required for the same degree of NO.sub.x reduction.
U.S. Pat. No. 5,284,636 (Goff et al.) is directed to another process employing phosphorus injection. However, Goff et al. are primarily concerned with the stabilization of heavy metals in fly ash. Disclosed is a system wherein the heavy metals stabilization is achieved by adding elemental phosphorus to the flue gas or the fuel to produce phosphorus oxides. Goff et al. suggest that simultaneous heavy metal stabilization and NO.sub.x reduction can be achieved by injection of phosphorus into the flue gas stream at any place upstream of a particulate separator device. Various injection sites are disclosed which range from the precombustion zone to a wet scrubber downstream of the combustion zone. The suggested temperatures for the injection of phosphorus range from 60.degree. F. (16.degree. C.) to 2500.degree. F. (1371.degree. C.). The temperature of the disclosed process is said to be limited by restrictions on materials of construction and not by the chemistry of the elemental phosphorus oxidation. Phosphorus is introduced either as an oxide species or directly as elemental yellow phosphorus. In the latter case, the elemental phosphorus is injected in the form of a water slurry including finely dispersed particles of solid elemental yellow phosphorus. The amount of phosphorus injected in the process is said to be dependent on the amount of fly ash produced and especially the concentration of heavy metal ions in the ash. The primary focus of Goff et al. is the use of phosphorus to produce oxides which when exposed to heavy metals such as Pb and Cd will interact to produce insoluble metal salts. Although the possibility of NO.sub.x reduction is mentioned, the specific reaction mechanism involved is not disclosed nor are the reaction conditions. The generation of ozone upon phosphorus injection into the flue gas and its involvement in the reduction of the NO.sub.x content of the flue gas is not even acknowledged. The ozone produced is not utilized at all in the stabilization of heavy metals. However, it is an extremely hazardous by-product if allowed to escape into the atmosphere and potentially more hazardous in the long run than the heavy metals entrapped. Goff et al. are silent on the effect of the ozone invariably produced upon injection of phosphorus into the flue gas. The survival times of ozone at the various injection sites disclosed and the possible quantities that could be released into the atmosphere through the stack are not discussed. Comparatively large amounts of phosphorus are used which renders the process uneconomical for NO.sub.x removal and, depending on the phosphorus injection site, the temperature of the flue gas at injection, and the method of particulate removal employed, may lead to potentially hazardous amounts of ozone being emitted to the atmosphere. Ozone is one of the most potent toxins known to man. Thus, a more economical and safe NO.sub.x removal process is desired.